Q: How would you summarize your study for a lay audience?

In short: We found that the defense protein "Resistin like molecule gamma" (Relmy), produced by neutrophils, punches holes into heart cells after a heart attack. This promotes dangerous, fast, and irregular heart rhythm and cell death in the heart.

The longer version: The most lethal complications of coronary artery disease are myocardial infarction (MI) and sudden cardiac death.

In MI, the blockage of a heart artery leads to insufficient oxygen supply to heart muscle cells (cardiomyocytes). This compromises their ability to maintain a stable rhythm and can give rise to a dangerous, unstable heart rhythms (arrhythmia) called ventricular tachycardia (VT) and ventricular fibrillation (VF).

VT and VF are both serious arrhythmias that can lead to sudden cardiac arrest and death within minutes. In VT, the heart beats very rapidly, but in a coordinated rhythm. In VF, the rhythm is chaotic and uncoordinated.

Most arrhythmias occur within 48 hours after MI and coincide with massive immune cell infiltration into the heart tissue. We were interested in how these immune cells may promote arrhythmia.

We found neutrophils that get recruited into the infarct (the area of dead tissue resulting from the cutoff of oxygen supply) in large numbers upregulate the gene "Retnlg," coding the protein resistin like molecule gamma (RELMy). We also found a comparable gene, "RETN," in human infarcted heart tissue. When we removed this protein from neutrophils in mice, the arrhythmia burden after MI was reduced 12-fold.

Q: What question were you investigating?

We were investigating the question of how neutrophils, a specific kind of immune cell, promote ventricular arrhythmia (a dangerous fast irregular heartbeat) after heart attacks. Cardiomyocytes as the main actors in arrhythmia are very well studied, but if and how immune cells can promote arrythmia is less clear. This work is important because ventricular arrhythmia is the most lethal complication after myocardial infarction. We need to understand better what promotes arrythmia to help us develop new antiarrhythmic drugs.

Q: What methods or approach did you use?

We used a plethora of methods to figure this out. For an initial understanding about which proteins in neutrophils might be important, we used deposited data on gene expression generated by single cell and spatial RNA-sequencing from mice that underwent myocardial infarction. But we also used data from human studies to find similarities in human tissue.

We also relied on confocal and super-high resolution microscopy in isolated mouse heart muscle cells that were treated with the labeled protein. Further, we deployed in vitro assays such as a liposome model and cell culture techniques to investigate the mouse and the human version of the protein to find out if they work similar.

Q: What did you find?

We found that after MI in mouse models, neutrophils upregulatethe expression of "Retnlg," the gene coding for RELMy. We also found that the human biological homolog "RETN," the genecoding for Resistin, was higher expressed in human infarcted myocardial tissue compared to non-infarcted tissue, similar to mice.

We saw that deleting the gene from bone marrow derived cells (such as neutrophils) and deleting the gene from neutrophils specifically significantly reduced incidents of ventricular arrhythmia in the mouse models.

Q: What are the implications?

The implications are that immune cells play a crucial role in sudden death and arrhythmia.

We should think about treating both the myocardial infarction both by quick recanalization of the vessel to restore oxygenated blood supply and also by targeting immune cells to mitigate the arrhythmic effects of the injury.

When we understand the underlying mechanisms better, we can pursue therapeutic targets that go beyond the broad immune suppression that is used today.

If we can treat targets more specifically, we can reduce unwanted side effects and unravel the full potential of immune modulation in cardiovascular disease.

Q: What are the next steps?

The next steps are to find a way to neutralize the harmful protein and test if this can reduce VT burden and infarct size. First in the mouse models, but, we hope, eventually also in humans.

We should gather more evidence about the significance of this protein in human disease. It is also interesting to see these findings have implications for other diseases with neutrophil recruitment and activation.

Authorship: In addition to Nina Kumowski and Matthias Nahrendorf, Mass General Brigham authors include Steffen Pabel, Jana Grune, Noor Momin, Kyle I. Mentkowski, Yoshiko Iwamoto, Yi Zheng, I-Hsiu Lee, Fadi E. Pulous, Hana Seung, Alexandre Paccalet, Charlotte G. Muse, Kenneth K. Y. Ting, Paul Delgado, Andrew J. M. Lewis, Vaishali Kaushal, Antonia Kreso, Dennis Brown, Kamila Naxerova, Michael A. Moskowitz, and Maarten Hulsmans.

Funding: This work was supported by grants from the Leducq Foundation, the National Institutes of Health (NIH grants HL155097, HL149647, HL142494, HL176359, NS136068, DP2AR075321); the Deutsche Forschungsgemeinschaft (DFG) Walter Benjamin Programm (491497342 and 530157297); the British Heart Foundation (FS/ICRF/24/26111 and RE/18/3/342140), and the NIHR Oxford Biomedical Research Centre.

Disclosures: Nahrendorf has received funds or material research support from Alnylam, Biotronik, CSL Behring, GlycoMimetics, GSK, Medtronic, Novartis, and Pfizer, and has received consulting fees from Biogen, Gimv, IFM Therapeutics, Molecular Imaging, Sigilon, Verseau Therapeutics and Bitterroot. Matthias and Wirth are employees of the company Abberior Instruments America, which commercializes the MINFLUX technology. Lewis is on the advisory board of Abbott, AstraZeneca, and Novartis. Pabel is employed by the Novartis Institute of Biomedical Research. Hayat is a cofounder and shareholder of Sequantrix GmbH and has received research funding from Novo Nordisk and AskBio. The remaining authors declare no competing interests.

"Sleep is often overlooked in postpartum recovery, but it is central to a mother's physical and mental health," said Moe Takenoshita, M.B.B.Ch., lead author of the study and a postdoctoral scholar in the department of anesthesia at Stanford University Center for Academic Medicine, Palo Alto, California. "Cesarean delivery in particular appears to increase the risk for severe pain and sleep disorders, which can lead to postpartum depression, thinking and memory problems, and fatigue, as well as disrupt bonding with their babies and relationships with family and friends."

The research team used both qualitative and quantitative methods. In the qualitative portion, they interviewed 41 mothers about their pain and sleep experiences after childbirth. Among them, 24 had vaginal deliveries, 11 had planned C-sections, and six had unplanned C-sections. Severe pain that affected sleep and everyday activities was reported by more than two-thirds of those who underwent C-sections (73% of planned and 67% of unplanned), compared to just 8% of mothers who had vaginal births.

The quantitative analysis drew from a national insurance database of more than 1.5 million mothers who gave birth between 2008 and 2021. The findings showed that mothers who had C-sections were 16% more likely to receive a new diagnosis of a sleep disorder (such as insomnia, sleep deprivation, or obstructive sleep apnea) within one month to one year after delivery, compared with those who gave birth vaginally.

Dr. Takenoshita advised that new mothers, particularly those recovering from C-sections, should take steps to manage pain effectively, since untreated pain can further disturb sleep. Additional strategies to support better rest include engaging in light physical activity when possible, sleeping when the baby sleeps, avoiding caffeine and alcohol later in the day, and winding down before bed through activities such as taking a bath or practicing deep breathing.

"About one-third of U.S. births are C-sections," said Dr. Takenoshita. "Those who are planning a C-section should understand that the procedure is linked to more severe pain after delivery and a higher risk of sleep disorders. Anyone having sleep problems during pregnancy or after childbirth should discuss their concerns with their physician, who can evaluate the issue, make recommendations and refer them to a specialist if necessary."